What makes Snap Casts (Interesting)

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What makes Snap Casts (Interesting)

Post Number:#11  Postby VGB » Mon Apr 16, 2018 8:47 am

Hi Dirk

This point is also interesting, it appears to show that changes to v-fly and v-loop do not follow each other immediately as described by the tension model. I think I see the delay also when I make the cast, do you think there are any other possible causes than a testing issue?

And whereas the rod tip’s acceleration reversal point coincides with the threshold of an acceleration plateau in markers 3 and 4, it precedes the threshold point of the loop’s acceleration plateau by one step...

This time step difference may be due to coarse steps used and requires calling for the magnification glass some time in future.


Regards

Vince
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What makes Snap Casts (Interesting)

Post Number:#12  Postby Dirk le Roux » Mon Apr 16, 2018 1:07 pm

VGB wrote:
So rod tip acceleration from here on is visibly impeded (by what?)


Dirk

I think that the rod tip acceleration is impeded by the combined inertia of the fly leg and rod tip. I think it’s another occurrence of the rod tip going backwards when we first accelerate the rod.


Hi Vince

I checked again and during the time after the instant shown, the rod is progressively less bent. It may be an occurrence of Graeme having begun the stop, so impeded by less input.

All the best, Dirk
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What makes Snap Casts (Interesting)

Post Number:#13  Postby Dirk le Roux » Mon Apr 16, 2018 1:22 pm

Graeme H wrote:I'm interested to see slightly positive rod tip acceleration at the highlighted frame in the last image posted above. The rod is still bent, indicating there is still some downward force, even if it's not full power. The graph indicates the tip is about to stop accelerating in the opposite direction to the force I'm applying, so its downward velocity is constant at that exact point. I guess it's a function of the image frame chosen at that very point, because the next point on the plot has positive acceleration (rod tip slowing.)


Hi Graeme

There may be some downward force still applied by the rod tip at that point, but as you can see from the plot, the tip's downward acceleration at that instant has already ceased and is now downward deceleration/upward acceleration. The downward force displayed by the bend there is likely smaller than the upward force at the loop at that time.

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What makes Snap Casts (Interesting)

Post Number:#14  Postby Dirk le Roux » Sun Apr 22, 2018 5:32 pm

VGB wrote:This point is also interesting, it appears to show that changes to v-fly and v-loop do not follow each other immediately as described by the tension model. I think I see the delay also when I make the cast, do you think there are any other possible causes than a testing issue?

And whereas the rod tip’s acceleration reversal point coincides with the threshold of an acceleration plateau in markers 3 and 4, it precedes the threshold point of the loop’s acceleration plateau by one step...

This time step difference may be due to coarse steps used and requires calling for the magnification glass some time in future.


Hi Vince

I checked using smaller steps (down from 5 to 3 frame steps) and the results for that instant now correlate more closely:

Plateau Loop Front.JPG
Rod tip zero acceleration vs. loop front acceleration

Plateau M3.JPG
Rod tip zero acceleration vs. Marker 3 acceleration

Plateau M4.JPG
Rod tip zero acceleration vs. Marker 4 acceleration

I stress again that the footage quality is quite good but not perfect, and as one uses smaller steps down to 1 frame per step, the data gets progressively noisier. The programme is very sensitive and small differences in eye-balled position of markers, with the imperfect video capture, influence the noise. Let’s call this the step catch 22, which can be overcome by very high resolution, high shutter speed, very high fps footage.

With the footage available, I found using 3-frame steps gives detail without noise getting too wild, but still some roughness especially where markers round the loop. I eye-balled marker positions as accurately as possible and anybody is welcome to ask for and check my files.

Regards,
Dirk
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What makes Snap Casts (Interesting)

Post Number:#15  Postby Dirk le Roux » Sun Apr 22, 2018 5:39 pm

Hi all

My previous posts showed roughly the very early instant at which the upward tension on the fly leg equals the downward force of the fly leg’s weight in a snap cast, as well as rod tip acceleration during the downward stroke and its interplay with acceleration at various point on the distributed line.

Now to leg velocities. For three different instances, I compare Y axis velocities of markers on the two legs at a region further away from the loop, as the step size with the available footage and angular changes near the loop, including where markers pass through the dolphin nose, produce noise (the step catch 22). A small degree of noise and thus roughness remains but let’s see.

While the loop front is on its way down:

a earth frame M2 M3 0.225s.JPG
Earth frame leg velocities, loop descending

The loop front’s travel velocity should be:

V_trav = (V_fl + V_rl)/2 = (3.32 + - 27.5)/2 = 12.09m/s

Let’s compare this answer with the loop front’s measured velocity:

a LF 0.225s.JPG
Loop front velocity, loop descending
a LF 0.225s.JPG (37.26 KiB) Viewed 591 times

Tracker can by set to use a marker’s reference frame. Setting as the loop front’s frame we can check how the loop’s calculated propagation velocity compares to the measured leg velocities in the loop front’s frame:

V_prop = (V_fl – V_rl)/2 = (3.32 - - 27.5)/2 = 15.41m/s

Compared to the measured leg velocities in the loop front frame:

a loop frame M2 M3 0.225s.JPG
Loop frame leg velocities, loop descending

Not bad.
Continued …
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Post Number:#16  Postby Dirk le Roux » Sun Apr 22, 2018 5:48 pm

Next the measured velocities while the loop is ‘frozen’:

b earth frame M5 M7 0.512s.JPG
Earth frame leg velocities, loop frozen

The loop’s travel velocity should be:

V_trav = (V_fl + V_rl)/2 = (10.57 + - 10.33)/2 = 0.12m/s

Compared to the loop front’s measured velocity:

b LF 0.512s.JPG
Loop front velocity, loop frozen
b LF 0.512s.JPG (38.38 KiB) Viewed 591 times

And the calculation for the loop front’s propagation velocity:

V_prop = (V_fl – V_rl)/2 = (10.57 - - 10.33)/2 = 10.45m/s

Compared to the measured leg velocities in the loop front frame:

b loop frame M5 M7 0.512s.JPG
Loop frame leg velocities, loop frozen

Again, not bad.

Continued …
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What makes Snap Casts (Interesting)

Post Number:#17  Postby Dirk le Roux » Sun Apr 22, 2018 5:54 pm

And measured velocities while the loop is climbing:

c earth frame M7 M8 0.675s.JPG
Earth frame leg velocities, climbing loop

The loop’s travel velocity should be:

V_trav = (V_fl + V_rl)/2 = (12.22 + - 6.07)/2 = 3.075m/s

Compared to the loop front’s measured velocity:

c LF 0.675s.JPG
Loop front travel velocity, climbing loop
c LF 0.675s.JPG (37.55 KiB) Viewed 591 times

And the calculation for the loop front’s propagation velocity:

V_prop = (V_fl – V_rl)/2 = (12.22 - - 6.07)/2 = 9.145m/s

Compared to the measured leg velocities in the loop front frame:

c loop frame M7 M8 0.675s.JPG
Loop frame leg velocities, climbing loop

The light blue track M 8 from the last sequence is that of the last marker before the leader. This track and that of the loop front during this marker’s rise were rather more erratic, which I don’t ascribe to noise from the footage and suspect may be related to the grass surface and its interaction with the thin line taper and leader. It would be interesting to check a similar cast with a smooth floor surface.

With the next series I will attempt an overview of velocity changes between the two legs in relation to the loop’s descent and eventual climb.

Regards,
Dirk
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What makes Snap Casts (Interesting)

Post Number:#18  Postby Dirk le Roux » Sun Apr 22, 2018 6:06 pm

Dirk le Roux wrote:The loop front’s travel velocity should be:

V_trav = (V_fl + V_rl)/2 = (3.32 + - 27.5)/2 = 12.09m/s


Hi all

Correction, the answer in the above calculation had a minus missing and it should read:

V_trav = (V_fl + V_rl)/2 = (3.32 + - 27.5)/2 = -12.09m/s

Regards,
Dirk
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What makes Snap Casts (Interesting)

Post Number:#19  Postby VGB » Sun Apr 22, 2018 6:22 pm

Thank you Dirk, I have used industry standard equipment up to 5000 FPS, you always get tracker noise. I think your M8 track might be because the legs are not perfectly parallel. I look forward to the discussion on why the loop starts to accelerate .

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